1. Field of Invention
The present invention relates to ultrasonic flow measurement and more particularly to an improved ultrasonic flow sensor that may be used alone or as part of a flow controller.
2. Discussion of Related Art
Ultrasonic flow sensing is often employed where a non-invasive approach to sensing fluid flow in a conduit is desired. Typical applications for flow sensors include, for example, the manufacture and/or processing of semiconductors.
Traditionally, a pair of ultrasonic transducers is employed where an upstream ultrasonic transducer transmits ultrasonic waves that are received by a downstream ultrasonic transducer, and the downstream ultrasonic transducer transmits ultrasonic waves that are received by the upstream ultrasonic transducer. The difference between the time of flight of the upstream ultrasonic wave and the time of flight of the downstream ultrasonic wave is determined and the velocity of the fluid flowing in the conduit may be calculated from the difference. Based upon the velocity of the fluid and other physical properties of the conduit and the fluid, the volume flow rate of the fluid may be determined.
Examples of ultrasonic flow sensors and controllers are described, for example, in U.S. Pat. Nos. 6,055,868, 5,974,897, and 3,575,050, and in commonly owned U.S. patent application Ser. No. 10/878,974, entitled “ULTRASONIC LIQUID FLOW CONTROLLER” (hereinafter “co-pending application”) filed Jun. 28, 2004, which is hereby incorporated herein by reference in its entirety.
The co-pending application describes systems and signal processing techniques that may be used with any ultrasonic flow sensor wherein the determination of the fluid flow rate is based upon a difference in time between a first ultrasonic signal that is propagated along a sensor conduit in the direction of fluid flow and a second ultrasonic signal that is propagated along the sensor conduit in a direction that is opposite to the direction of fluid flow.
Although the technology described in the co-pending application provides many advantages over conventional ultrasonic flow measurement systems and signal processing techniques, reliably getting ultrasonic energy into and out of fluid flowing in an ultrasonic flow sensor continues to be a difficult problem. Further, it is often difficult to measure very small flow-induced upstream to downstream time differences, particularly at relatively low rates, such as, for example, between 5 and 50 mli/min (and consequently relatively small internal sensor conduit diameters). Still further, where the acoustic impedance of the material from which the sensor conduit is formed is relatively closely matched by the acoustic impedance of the fluid flowing therein, one typically cannot rely on the acoustic energy imparted to the fluid being internally reflected by the material of the sensor conduit. In many such systems, the material from which the sensor conduit is formed may strongly attenuate higher ultrasonic frequencies (e.g., those above 1 MHz), such that the acoustic wavelengths employed are comparable to the inner diameter of the sensor conduit.